Piezoelectric device



ApriE 10 1945. H. s. BAERWALD 2,373,445

PIEZOELECTRIC DEVICE Filed Jan. 18, 1945 6 Sheets-Sheet 1 IN VEN TOR. HAP/f 6. BAERWALD A9155 1945- H. G. BAERWALD 2,373,445

' PIEZOELECTRIC DEVICE Filed Jan. 18, 1943 6 Sheets-Sheet 2 FIRM - '5 %fi\\\\\\\\\\\fi\ x 4 I g INVENTOR.

HAN! 6f BAERW/JL D ATTORNEY April 1945 I H. s. BAERWALD 2,373,445

PIEZOELECTRIC DEVICE Filed Jan. 18, 1943 6 Sheets-Sheet 3 INVENTOR. HA/Yf G. BAERWALD ATTORNEY April w, 1945.

G. BAERWALD PIEZOELECTRIC DEVICE 40 t e e h s .H m E S m m 6 Filed Jan. 18, 1943 INVENTOR. HANI a. BAERWALD A T TOR/Yfy April y 1945 H. s. AERWALD 2,373,445

PIEZOELECTRIC DEVICE Filed Jan. 18, 1945 6 Sheets-Sheet 5 4 INVENTOR. 2 HA/l/ a BAE/ih/ALD BY E A T TOR/YD Avail 1945a H. G. EAERWALD PIEZOELECTRIC DEVICE Filed Jan. 18, 1943 6 Sheets-Sheet 6 IN VEN TOR. H/J/Yf 6. BAfHM AL D a fi w I; ATTORNEY Patented Apr. 10, 1948 2373.445 PIEZOEEC'IBIO DEVICE HansG.Baerwald,Clev

slgnorto'l'heBrush Cleveland. Ohio, a

clan! Heights, Ohio, as Development Company, corporation of Ohio Application January 18, 1943, Serial No. 472,!"

' 10 Claims. (01. 171-327) This invention (Brush Docket No. 879) relates to piezoe ectric devices and. more particularly, to piezoelectric transducer elements of the multiplate type adapted to generate electric potentials representative of bending or twisting forces applied thereto and, conversely, to bend or twist in response to applied electric potentials.

Piezoelectric devices of the general type to which this invention pertains are described in the United States reissued patents to Charles B. Sawyer, Re. 20,213 and Re. 20,680. The devices are now well-known under the trade-mark Bimorph," and are widely utilized in transducers such as microphones, loud-speakers, telephone receivers, vibration pickups and the like.

It is not to be inferred from the citation of the Sawyer patents, however, that the invention is limited to "Bimorphs fabricated from Rochelle salt. The phrases "piezoelectric matter and "piezoelectric material, on the contrary, wherever employed herein are to be understood as embracing such crystals as beryllium sulphate, beryllium oxide, primary ammonium phosphate, primary sodium or potassium phosphate and isomorphous mixtures of phosphates, as well as certain of the arsenates, tourmaline, etc, thus embracing all material from which plates or sections, exhibiting piezoelectric properties, may be cut.

The main purpose of the Bimorph construction or, more generally speaking, of multi-plate combinations of crystal sections, each of which acts as an expander or as a shear element, is to obtain mechanical advantage. With the exception of supersonic applications, in the overwhelming majority of applications the crystal element works into some form of mechanical load, the impedance of which is many times smaller than that of a pure crystal slab. In such applications, the Bimorph" or multi-plate element gives the required step-down of force, 1. e., it increases the movement available for a given voltage. For practical uses, the mechanical advantage gained is of the order of from 1:4 to 1:50. It is well-known that the conventional "Bimorph" is a compromise that suffers from some shortcomings, mainly because of the fact that different crystal sections work under difierent elastic loads and are non-uniformly restrained, the ,restraint decreasing from the cemented plane outward toward the electroded surfaces. In a Bimorph" of the bender type, for example, no matter what the piezoelectric material, those parts near the cementing plane contribute but little to the'bending couple although a considerable part of the available overall voltage is dropped in them. This is particularly true of Bimorphs" fabricated from X-cut Rochelle salt sections which, under restraint, have a higher dielectric ,stiflness than the peripheral parts that are far less restrained.

The highest eillciency of a piezoelectric transducer element can be realized only if the distribution of the electro-mechanical quantities, such as stress, strain, electric ileld and dielectric displacement, is homogeneous over the crystal 7. volume. This desideratum is realized in "straight expander" or "pure shear plates. In order to obtain substantially equivalent efllciency in a compound element, it follows that the mechanical advantage must be secured externally thereof.

In the United States patent to Wente, 1,438,974, a compound construction is disclosed which offers the foregoing desiderata when the external forces are substantially zero. It is obvious, however, that the disclosed construction would be less eflicient than a conventional "Bimorph" if an attempt were made to utilize it in a transducer, such as a loud-speaker or the like, particularly at elevated frequencies. The practical disadvantages of the Wente device are a result of construction difllculties and of the type of external motion obtained. with respect to the first, hinges of sturdy construction and long lever arms would be, required, the former being diflicult to carry out without play and both of which would involve relatively large dead masses and concomitant ,lowering of the natural frequency of the assembly. With respect to the second, the resultant motion, in the cited apparatus, is angular about an axis parallel to thewidth dimension of the sections. In order to avoid non-homogeneous loads on the sections, the mechanical utilization of the angular motion requires symmetrical mechanical loads; if it is desired to drive only one load, additional mechanical devices, introducing additional mass, must be employed.

Having in mind the foregoing, the primary object of this invention is to provide an improved piezoelectric element of the multi-plate type.

Another object is to provide a workable and practicable compromise between structures of the ideal type and structures of the conventional type now manufactured and sold by The Brush Development Company under the trade-mark Bimorph.

Another object is to provide a compromise piezoelectric structure of the multi-section type that shall exhibit substantially the same variety of motion as that of the conventional Bimorph.

In the manufacture of Bimorphs" and similar multi-section piezoelectric elements fromsections of Rochelle salt, the plates are usually cemented together by melted Rochelle salt. Suchcement asvaus some dlfiiculty is experienced in applying electrodes to the inner surfaces of the hollow.

The novel features of the invention are set forth with particularity in the appended claims.

does not consist, in reality, of Rochelleflsalt'but merely has the gross composition thereof because when the salt is melted in its own, water oi crystallization it disassociates into its component salts for the reason that the combination is unstable" with less than four molecules of water per molecule thereof and it does not spontaneously reform. The sodium and potassium components have quite difierent solubilities; the cement, therefore, is an undercooled solution of the two in unequal parts because of the precipitation of one of them. Such cement solidifies suddenly, because of undercooling, and it cannot be expected thatthe solidified mass, which probably is not homogeneous, will exhibit as good mechanical properties as the crystal sections that it joins together, especially with respect to shear. An additional disadvantage, therefore, accrues from the manner in which the sections constituting a Bimorph are usually fastened together, inasmuch as the cement is subjected to a. pure shear stress, i. e., one to which it is but poorly resistant. As a consequence, partial yielding of the cement and, which is worse, non-reversible deformation thereof, or mechanical hysteresis may occur and detract from the dynamic eifi- Y ciency of the device. This disadvantage is augmented in the case of thin sections, where the thickness of the cement layer constitutes an appreciable portion of the over-all thickness of the Bimorph. A considerable amount of nonreciprocity is caused, it would seem, by the presence of the cement.

Another object of the invention, accordingly,

is to provide a multi-section piezoelectric transducer element substantially devoid of detrimental mechanical hysteresis and free from non-reciprocity.

Another object is to provide a piezoelectri transducer element, of the multi-plate type, wherein the minimum amount of cement or the like is employed for afflxing the plates each to theother. 1 1

The foregoing objects and other objects that will become apparent hereinafter are attained,

inoneembodiment of the invention, by connecting together twoelongated crystal slabs adia-' centtto their ends only. to constitute a bender 1 or aifjtwister orby connecting together the corners onlyzof two otherwise spaced apart square sectionstaj;Preferably; the contacting portions of theissections are given such a contour that the cement ntherebetween is subjected to normal stresses rather than shear stresses and the sections themselves are so shaped'that spot? stress concentrations are substantially avoided.

The objects of the invention may also be attained by hollowing out the center of -a, single 7 rectangular crystal section, but that requires somewhat dimcult machining operations and The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood irom the following description of certain specific embodiments, when read in connection with the accompanying drawings, in which:

Figure l is a view in perspective oi a Rochelle salt transducer element, fabricated according to the invention, whereof the plates, or sections are integral with each other:

Figure 2 is a view in perspective of a transducer element analogous in contour to the element 1 shown in Figure v1, formed from two individual plates of piezoelectric matter;

Figured is a view in perspective 0! one oi the plates comprised in the element in Figure 1, illustrating an inner electrode in place;

Figure 4 is a .view in perspective of a still iurther embodiment oi the invention;

Figure 5 is a plan view 02 a "bender" constructed according to the invention;

Figure 6 is a vertical cross-sectional view of the device shown inFlgure 5, taken along a line corresponding to the line VI-VI in that figure;

Figures 7, 8, 9 and 10 are side views in elevation, exemplifying alternative embodiments of the invention;

Figure 11 is a view in perspective of a twister element constructed according to the invention;

Figures 12 and 13 are plan views of the top and bottom plates, respectively, of the transducer element shown in Figure 11;

Figure la is e. viewin perspective of an alternative form of twister element fabricated according to the invention;

' ing to the invention, the corners of which are r interlocked;

Figure 17 is a view in perspective oi a single one of the plates of the element shown in Figure 16;

Figure 18 is a plan view of the plate shown in Figure 1'1;

Figure 19 is a view in perspective of a plate provided with corner projections adapted to interlock with the corner projections of a similar plate to form a twister" element of the type shown in perspective in Figure 20.

For the purpose of facilitating the description of certain of the many alternative forms that a transducer element may assume when constructed according to the invention, the term duomorp will be employed to distinguish them from elements or the conventional type such as are shown in the Sawyer patents cited. The term is to be understood as including all transducer elements constituted by at least one plate of piezoelectric matter, integrally or otherwise connected only along two of its edges or at its corners only to the edges or comers. respectively, of another plate of piezoelectric material and as also including elements constituted by at least one plate of such material connected at its extremities to a flexible restraining plate of non-piezoelectric material such, for example, as berylliumcopper. There is no intention, however, of limitthe specific embodiments illustrated and described hereinafter nor of excluding other embodiments, within the spirit and scope of the invention, that will be apparent to those skilled in the art.

The invention may be practiced, as shown in Figure 1 of the drawings. by hoilowlng out the center of a single section. i cut from a mother crystal of piezoelectric material to provide a duomorph somewhat resembling a flattened cylindex. The original section may be rectangular and it may be so out, as illustrated, from a crystal of Rochelle salt that its major faces are perpendicular to the X axis of the crystal and its ends intersect the Y and Z axes at an angle of 45. The inner and outer major faces of the crystal element may be provided with electrodes 3 by spraying colloidal graphite onto them as disclosed in the United States patent to A. L. W. Williams No. 2,106,143, or by affixing metallic foil thereto, the electrodes stopping short of the dead ends in order to restrict the electric field to the active portions of the device. Only two of the electrodes 3 are shown in the drawings, but it is to be understood that the invisible inner and outer faces are also electroded.

The particular device shown in Figure lis a duomorph of the "bender type, adapted to the same uses as the Bimorphs shown in the Sawyer patent and to many other and well-known uses. For example, one end of the device may be provided with fixed clamping means (not shown) and a connection may extend from the free end, in the direction of the X-axis, to the apex of a loud-speaker cone or the like (also not shown). If the inner electrodes are connected to one terminal of a source of alternating potential and the outer electrodes connected to the other terminal, one plate, apart from bending, will expand while the other plate will contract in a direction parallel to the long dimensions thereof with the result that the duomorph flexes as a whole in a plane containing the X-axis and bisecting the angle between the Y and Z axes, as is desired.

In many figures of the drawings, sharp inner corners and sharp outer end edges are shown for purposes of convenience. It is to be understood, nevertheless, that in actual traiisducers, constructed according to the invention, such edges and corners, preferably, are rounded-oi! to minimize stress concentrations.

In the event that it is not feasible to fabricate a duomorph by hollowing out the interior of a single crystal section, substantially the same effect may be obtained by disposing two separate sections in face-to-face spaced apart parallel relatlon and by aiiixing the opposed faces adjacent to their ends, to each other as rigidly as possible. For example, one major face of each section 'l as shown in Figures 2 and 3, may be provided with two integral ridges or ledges 9, that lie substantially arallel to the ends thereof, and extend in a direction substantially perpendicular to the said faces. The sections may be assembled, as shown in Figure 2, with the end ridges in contact with each other and cemented together,

The invention may also be practiced, as shown in Figure 4 of the drawings, by disposing two electroded crystal sections II in face-to-face spaced apart parallel relation, the spacing being maintained by cementing one pair of end edges into appropriate channels in a connecting element l3 and by interposing a spacer between the inner. faces of the sections at the opposite end, to which spacer the said faces are rigidly cemented. Substantially any rigid material may be utilized for the spacer, provided the crystal sections may be caused to adhere firmly thereto; the crystal substance, itself, may .be employed if desired. An intermediate transverse brace H or a plurality of such braces may be added if the sections are very thin and close together, whereby they may exhibit a tendency to buckle.

The channeled connecting element i3, preferably, is fabricated from a material that is relatively light yet is sufficiently rigid that the edges of the channels cannot deform to an appreciable extent when the device is in operation. Beryllium-copper would seem to be one of the most desirable materials to employ.

The duomorph illustrated in Figure 4 is of the bender type, adapted to impart substantially linear motion to a bar E8, or the like, affixed to the connecting element, as indicated by the arrows. A twistcr," or shear type, transducer may b formed in a similar manner by utilizing sections the edges of which, respectively, are perpendicular to the Y and Z axes of the mother crystal of Rochelle salt, as disclosed in the Sawyer patents cited. Piezoelectric materials other than Rochelle salt may also be employed, provided, of course, that the orientation of the faces and edges of the several sections with respect to ,the crystallographic axes thereof is such as to secure the desired mode of fiexure.

A further embodiment of the invention is illustrated by Figures 5 and 6 of the drawings. in that modification, only the lower section 1 is provided with end ridges 9, or fillets, and each ridge is stepped to accommodate, the corresponding ends of a plain upper section 2| that are cemented thereto. The cement is subjected partly to shear stresses and partly to stresses of the compression-tension type.

There are numerous other ways in which a duomorph, analogous to the embodiment of the invention exemplified by Figures 5 and 6, may be constructed. A few of such alternative constructions are illustrated by Figures 7, 8, 9 and 10, which are side views in elevation of the said constructions, the electrodes 3 not being illustrated. It is believed that Figures 7, 8, 9 and 10 are sufllciently self-explanatory to obviate the necessity of detailed description. Attention, however, is particularly called to Figures 8, 9 and 10 which show embodiments of the invention wherein the cement between the sections is subjected to compressional-extensional stresse instead of to pure shear stresses, and wherein slip and hinge-action at the cemented ends are reduced to the minimum. After assembly, surplus dead material'may be trimmed from the ends of a duomorph, as indicated by the dotted lines in Figures 9 and 10.

A duomorph of the twister type may be constructed according to the invention, and as shown in Figures 11, 12 and 13, by providing two square sections, the corners of the lower section 23 having integral stepped projections 25 and the corners of the upper motion 27 being so cut A still further embodiment of the invention is illustrated by Figures 16,17 and 18. In that embodirnent, each section is provided with a plurality of integral corner-projections 29, triangular in plan instead of square, which projections definitely interlock to provide extremelyrigid connections. cement interposed between the projections are partly shear and partly compressional-extensional.

Another specific embodiment of the invention, wherein the corners oi a duomorph are rigidly interlocked, is exemplifi'edby Figures 19 and 20. In that form, each corner of each section is'provided with an integral projection 3! that interlocks with the corresponding projection of the other section and also fits into -a channel 33 provided for its accommodation. The duomorph so formed is eflicient in operation although, as is obvious, it is more difll'cult to manufacture than other forms herein described.

Although a number of specific embodiments of the invention have been shown and described, it is realized that many additional modifications understood that the invention applies also to In use, the stresses imposed upon the transducers adapted to produce electrical potentials in response to applied forces and that it is not limited to sections cut from Rochelle salt. The invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

What is claimed is:'

1. In an electrical transducer device, a flexible plate of piezoelectric matter, a flexible restraining plate disposed in iace-to-face substantially parallel relation and spaced apart from said piezoelectric plate throughout the major portion of their extent, first connecting means rigidly connecting one end of the piezoelectric plate to one end of the flexible plate, and second connecting means spaced from said first connecting means and rigidly connecting a second end of said piezoelectric plate to a second end of said flexible plate, the rigidity of said two connections and the flexibility of said two plate being so related that upon a change in length of the piezoelectric plate with respect to said flexible plate between said two connection means a force is transmitted through at least one of said connection means in a direction substantially perpendicular to the major planes of said two plates whereby the said flexible restraining plate is bent.

2. In an-electrical transducer device, a flexible plate of piezoelectric matter, a flexible restraining plate disposed in face-to-face substantially parallel relation and spaced apart from said piezoelectric plate throughout the major portion of their extent, first connecting means rigidly connecting one end of the piezoelectric plate to one end of the flexible plate, and second connecting means spaced from said first connecting means and rigidly connecting a second end of said piezoelectric plate to a second end of said flexible plate,

, the said two connecting means being sufliciently rigid that upon a change in length between said two connection means or the piezoelectric plate with respect to said flexible plate one of said two connection means maintains the positional relationship of the first connected end of the piezoelectric plate with respect to the first connected end 01 the flexible plate and the other connection means maintains the positional relaaavaeas nected end of the flexible plate and a force is transmitted through at least one of said two connection means in a direction substantially perpendicular to the major planes or said two plates whereby a bending action is induced in said piezoelectric and flexible plates.

3. In an electrical transducer device, a first flexible plate of piezoelectric matter, a second flexible plate of piezoelectric matter disposed in face-to-face substantially parallel relation and spaced apart from said first plate throughout the major portion of their extent and-so oriented with respect to the crystallographic axis or the piezoelectric material that one plate tends to expand in one direction while the other plate tends to contract in said same direction, first connecting means rigidly connecting one end of the first piezoelectric plate to one end or the second piezoelectric plate, and second connecting means spaced from said first connecting means and rigidly connecting a second end of said first plate to a second end of said second plate, the rigidity of said two connections and the flexibility of said two plates being so related that upon a change in length of one of said piezoelectric plates with respect to the other of said piezoelectric plates between said two connection means a force is transmitted through at least one of said connection means for bending said two piezoelectric.

plates.

4. In an electrical transducer device, a first flexible plate of piezoelectric matter, a second flexible plate of piezoelectric'matter disposed in face-to-face substantially parallel relation and spaced part from said first plate throughout the major portion of their extent and so oriented with respect to the crystallographic axis or the piezoelectric material that one plate tends to expand in one direction while the other plate tends to contract in said same direction, first connecting means rigidly connecting one end of the first piezoelectric plate to one end of the secticnship of the second connected and or the 0nd piezoelectric plate, and second connecting means spaced from said first connecting means and rigidly connecting a second end of said first plate to a second end of said second plate, the said two connecting means being sufiiciently rigid that upon a change in length between said two connection means of either of the piezoelectric plates with respect to the other piezoelectric plate one. of said two connection means substantially maintains the positional relationship of the first connected ends-of the two piezoelectric plates with respect to each other and the other connec tion means maintains the positional relationship of the second connected ends of the piezoelectric plates with respect to each other, and a force is transmitted through at least one of said two connection means in a direction substantially perpendicular to the major planes of said two plates whereby a bending action is induced in said piezoelectric plates.

5. A device as set forth in claim 1, further characterized in this; that said plate of piezoelectric matter and said flexible restraining plate are rigidly connected together at only their four corners.

6. A device as set forth in claim 1, further characterized in this; that said plate of piezoelectric matter and said flexible restraining plate are rigidly connected together substantially throughout the length of two spaced apart parallel edges.

7. A device as set forth in claim 1, further characterized in this; that said plate of piezoelectric matter and said flexible restraining plate are rigidly connected together at only their four corners, and that said plate of piezoelectric material is a twister plate whereby the said flexible restraining plate is bent about two axes of curvature.

8. A device as set forth in claim 2, further characterized in this; that said plate of piezoelectric matter and said flexible restraining plate are rigidly connected together at only their four corners.-

9. A device as set forth in claim 2, further characterized in this; that said plate of piezoelectric matter and said flexible restraining plate are rigidly connected together substantially throughout the length of two spaced apart parallel edges. I

10. A device as set forth in claim 2, further characterized in this; that said plate of piezoelectric matter and said flexible restraining plate are rigidly connected together at only their four corners, and that said plate of piezoelectric material is a twister plate whereby the said flexible restraining plate is bent about two axes of curvature.

11. A device as set forth in claim 3, further characterized in this; that said first and second flexible plates of piezoelectric matter are rigidly connected together at only their four corners.

12. A device as set forth in claim 3, further characterized in this; that said first and second flexible plates of piezoelectric matter are rigidly connected together substantially throughout the length of two spaced apart parallel edges.

13. A device as set forth in claim 3, further characterized in this; that said first and second flexible plates of piezoelectric matter are rigidly connected together at only their four corners, and that said plates of piezoelectric material are twister plates whereby the said flexible plates are bent about two axes of curvature.

14. A device as set forth in claim 4, further characterized in this; that said first and second flexible plates of piezoelectric matter are rigidly connected together at only their four corners.

15. A device as set forth in claim 4, further characterized in this; that said first and second flexible plates of piezoelectric matter are rigidly connected together substantially throughout the length of two spaced apart parallel edges.

16. A device as set forth in claim ei, further characterized in this; that said first and second flexible plates of piezoelectric matter are rigidly connected together at only their four corners, and that .said plates of piezoelectric material are twister plates whereby the said flexible plates are bent about two axes of curvature.

17. A device as set forth'in claim 2, further characterized in this; that said plate of piezoelectric matter and said flexible restraining plate are cemented together, the contours of the contacting portions of the plates being such that during utilization of the device the cement is subjected to stresses that are predominately of the compressional-extensional type and stresses of the shear type in the cement are minimized.

18. A device as set forth in claim 4, further characterized in this; that said two plates of piezoelectric matter are cemented together, the contours of the contacting portions of the plates being such that during utilization of the device the cement is subjected to stresses that are predominately of the compressional-extensional type and stresses of the shear type in the cement are minimized.

HANS G. BAERWALD. 

